Tascam 38 transport weirdness

A

Abominable ID

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Help!!!

I took my left reel motor out to clean the brushes/communicator, and put it back in. Now the transport is doing all kinds of strange things like FF when I push PLAY, and no REW at all. Also the STOP button only works if I push FF or REW, not after pushing play. In fact, when I push STOP after PLAY, both reels start up together and go faster. It's completely bizarre because the machine was working perfectly before the motor was removed and prior. Now it wont even REW and none of the functions correlate with the buttons. I'm absolutely stumped as to what could be wrong. The only thing I disconnected was the motor wires which I soldered back exactly as they were. Although the machine was off, could I have blown some sort of chip? Or maybe one of the transistors that are screwed to the cooling fins? I really appreciate any help out there...

-Ash
 
Have you got the manual? The '32 manual contains a detailed description of the M54410P transport logic chip, including a truth table for inputs and outputs from the chip (U519 on the '32) and a state transition table as well. If it were my deck, that's where I'd start.

Use a 'scope or logic probe to try and see what logic levels are actually going into it, and what logic levels are coming out. If the inputs are garbage, the problem must be before the chip. If the inputs are fine but the outputs are nonsense, the chip is probably shot. If the inputs and outputs are both sensible, the problem must be after the chip, presumably whatever converts the TTL signals to something the motors can use.
If the chip is malfunctioning, replacements are available last I looked, since it's a generic part also used in cassette decks. Worst case, the manual contains enough detail to build an equivalent daughterboard from 7400 logic. But unless it's socketed, I wouldn't just replace it to see what happens.
 
That's a really good suggestion but not the problem. I actually fried that chip while making my own remote control. I ordered two new ones (wasn't easy to find). So yeah the first thing I checked was to pop the other chip in there and I have exactly the same problems. Do you happen to know if those two transistors attached to the cooling fins are responsible for switching the motors direction? The most confusing part is that I didn't touch anything except those two transistors and the motor, and now all the erratic behavior...it's as if it's possessed!
 
First, are you testing the machine with tape actually loaded on it when you get these abnormal results? If you're just defeating the tape tension sensor with no reels loaded, it's normal for both reel motors to engage, one of them is given reduced power so that it provides back-tension and that will make them both spin.

That said, I have a theory as to what's happened. On the '32, there is a cluster of logic gates which take the output from the M54410P, and perform some additional operations on it. This is then fed down to the Darlington drivers which power the motors (I suspect one of those is the big transistors on the cooling fins, but the schematics don't give the physical location unfortunately, and the '38 might be slightly different anyway since it moves twice as much tape).

But. There are three inputs to this logic circuit: FF enable from pin 15 of the M54410P, REW enable (pin 13 of the same) and also pin 7 of U516, which is a comparator chip giving the tape speed as a high/low logic level.
That third input comes from a set of yet more logic gates fed from the motion sensor circuit, which is driven by a rotary encoder which is attached to one of the motors.

If that's the motor you removed, you need to check that the encoder is still in place, that it's still rotating when the motor turns, and that the vanes are still interrupting the photosensors. If it's not doing all of those things, you're going to get some very, very strange results from the reel motor control logic.
 
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No, and actually the problem got worse. It started with the left reel motor (not the one with the encoder) after I removed it and cleaned the communicator. After some more testing with and without tape, the motors stopped working altogether. However, the brake solenoid still works as it should, and the capstan motor as well. This seems to me to obviously be the circuit that controls the motors. Remember that the first problem was that it wouldn't go into REW. I also found it strange because when I removed the motor, there was no power to the machine, so I don't think I could have shorted anything! I am completely baffled by this and will have to take the machine in for service. I have ordered a used motherboard off of Ebay which I am tempted to swap it in and see if it works again...but the only problem there would be all the tape tension settings might not be the same as my machine. I guess Im worried that I might do more damage, plus I won't know what is causing the current problem.
I think it must be one of the logic chips that controls the motors. I have searched online and can't really find anything. I'm in pretty deep here, but I made a commitment that I'm gonna stick with the 38 until I'm rich and can buy something better. Any ideas are more than welcome, although I can seem to find anyone who has had this problem before!

-Ash
 
When you removed the front panel, did you disconnect the plug and socket on the flexible cable that links the keyboard to the Control PCB?

I have a Tascam 34 on that the plug and socket is shown on "Exploded View 2" as items numbered 6 & 7.

I don't think this is possible or likely. but If so, have you somehow reconnected it incorrectly?

Also, do you get the same errors with your remote control?
 
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